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The importance of maintaining the purity of rheological terms and using them only when measurements and definitions are in accordance with the accepted rheological definitions is pointed out. It is shown that if rheological terms and methods of measurements and analyses are used correctly, they can be employed in food texture studies even though conditions upon which the theory is based are not fully met. It is proposed that the term “modulus of elasticity” be replaced by the term “modulus of deformabtiity” which considers both recoverable and unrecoverable deformations that often take place when a food sample is subjected to even very small strains. It is also submitted that mechanical properties reflecting the behavior of the material under small strains do not necessarily correlate significantly with mechanical properties which reflect the yielding and fracturing or failure of the material. Published data are examined to test the validity of this hypothesis. In general, correlations between well‐defined failure type instrumental tests and panel tests are good, whereas correlations between small‐strain instrumental tests and panel tests are poor.

SUMMARY A demand currently exists for information on the mechanical behavior of foods and agricultural products during handling, processing and quality evaluation. Many of the older subjective tests have now been replaced by various techniques from which empirical and partially objective measurements may he obtained; however, the majority of these measurements are not well‐defined in terms of accepted physical constants, thereby making their interpretation difficult. To provide consistency between various investigations, all mechanical properties may be evaluated in terms of common engineering parameters as the first approximation. From the point of view of mechanics, many products such as fruits, vegetables, and cereal grains in their natural state may be considered as convex bodies for which classical solutions can be applied for evaluation of compression, shear and tension properties. In this paper previous and current investigations in which the engineering approach to mechanical property evaluation has been used are reviewed. In all of these investigations, the common methods for product testing are hydrostatic and uniaxial compression. Instrumentation and techniques are described for obtaining and interpreting data from these tests in terms of physical constants. The mechanical behavior of most agricultural products is time dependent. Therefore, characterization of mechanical behavior requires the application of viscoelasticity principles in which both viscous and elastic responses are combined. The fundamental principles of viscoelasticity are presented briefly and analogies are developed for relating the observed behavior to well‐established mechanical systems. To illustrate the application of the engineering approach to mechanical property evaluation, a study on McIntosh apple fruits is described. This study demonstrates that many of the principles of classical mechanics are applicable to selected agricultural materials.

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The purpose of this paper is to point out the importance of understanding what the food materials are like, from the solid mechanics viewpoint, and how they fail under stress and strain. The difficulty in defining the threshhold of failure for solid foods is discussed, and suggestions are made with reference to three selected food materials. The importance of relating the mechanical behavior to structural changes in the plant and animal materials is pointed out. A comprehensive illustrated review of the stress‐strain‐time effect on solid food materials is given in a tabulated form. The review includes the type and location of stresses causing failure, the type of loading which has been shown to be more meaningful in characterization of the material, and failure‐related findings pertaining either to test specimens taken from the material or to the intact material in its natural shape. It is concluded that a basic approach requiring some mathematical manipulation and a knowledge of solid mechanics, a t least to apply the known theories i f not to develop new ones, is a prerequisite for understanding the mechanics of solid foods. Such knowledge is expected to be applied to many problems involving food handling and processing. The use of rheology in texture evaluation of food is only one of the many possible applications.